An evaporated fuel processing apparatus has been widely used, which is configured to: temporarily adsorb evaporated fuel in a canister for preventing the evaporated fuel from flowing to the outside, wherein the evaporated fuel occurs in a fuel tank of a vehicle, and the canister is made of an adsorbent such as activated carbon; and thereafter purge a fuel component from the canister by introduction of fresh air and introduce the fuel component into an intake air line of an internal combustion engine while the internal combustion engine is operating.
A patent document 1 discloses an evaporated fuel processing apparatus which includes a blocking valve in a passage between a fuel tank and a canister, and is configured to adsorb evaporated fuel from the fuel tank into the canister, basically only during fuel filling. This system is configured to maintain the fuel tank in hermetic state by the blocking valve when a vehicle is stationary and no fuel filling is being performed, and thereby more reliably prevent evaporated fuel from flowing to the outside.
The evaporated fuel processing apparatus according to patent document 1 is provided with a diagnostic apparatus for diagnosing whether or not the blocking valve is in a failed open state in which the blocking valve is fixed in open state and cannot be closed. This diagnostic apparatus includes a negative pressure pump connected to a drain port side of the canister, and depressurizes an inside of a system by the negative pressure pump with controlling the blocking valve into closed state in suitable timing while the vehicle is stationary, and diagnoses whether or not the blocking valve is in a failed open state, based on a pressure change in a fuel tank side region or a canister side region at that time.
However, the conventional diagnostic apparatus is configured to perform the diagnosis after operation of the vehicle is stopped. Accordingly, it is necessary to monitor pressure changes with the system of the evaporated fuel processing apparatus maintained in hermetic state, so that the diagnosis cannot be successively performed while an internal combustion engine is operating.
Moreover, the conventional diagnostic apparatus requires a pressurization/depressurization means such as an electric pump, and therefore is not applicable to an evaporated fuel processing apparatus provided with no pressurization/depressurization means.